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Types of transformer fluid tests

Types of testing

A good analogy for equipment fluid analysis is an annual health physical. The physical usually involves a blood sample, where the doctor may explain how the results show immediate or long-term health impacts. Fluid analysis can also identify issues that shorten equipment life even before they manifest in external signs.
Transformer routine insulating fluid tests: Routine lube oil/hydraulic fluid tests:
Dissolved Gas Analysis (DGA) Viscosity at 40°C
Screen or physical properties Acid / Base Number
Dissolved Moisture ICP-AES Spectroscopy
Optional tests for further investigation Particle Count
Furan / Methanol Fourier Transform infrared Spectroscopy (FTIR)
Particle count Ferrous Density
ICP-AES Spectroscopy Membrane Patch Colormetric (MPC)
Fire point Linear Voltammetry for Additive Content

Screen testing

Screen testing evaluates the physical, electrical and chemical attributes of the transformer oil. Each test is an indication of how suitable the insulation liquid is for service. HSB’s screen tests include: Dielectric breakdown, Power factor, Interfacial tension, Acidity, Color, Specific gravity.

Dielectric breakdown measures the breakdown voltage of an insulation liquid. This test also serves as an indicator of the presence of other contaminating agents such as sediment, moist paper fibers and conducting particles. ASTM D-877 was the method of testing dielectric breakdown until recently. The ASTM D-1816 uses rounded probes at a specific gap with a stirring action to produce results that are more sensitive to moisture and particle contamination. The D-1816 can produce lower breakdown voltage results that may still be within standard guidelines.

Power Factor measures the amount of energy dissipated as heat. This test helps reveal the quality and integrity of the insulating liquid and can be used to determine when a filtering or change of transformer liquid is needed. The power factor of overheated or contaminated insulating fluid will increase and can give the based for additional investigation.

Interfacial tension determines the presence of polar compounds, which are considered to indicate oxidation contaminants or deterioration from the transformer construction materials. Elevated moisture content will reduce the insulating fluid IFT and may indicate the need for fluid processing.

Acidity measures the acid levels in the transformer liquid. As acid levels increase, the quality of the oil decreases, causing dielectric loss, corrosion, and thermal problems from sludge.

Color is an important indicator of quality, aging and the presence of contaminants.

Specific gravity verifies the weight to provide the type of insulating fluid.

Moisture testing

This test uses the Karl Fisher method to determine the amount of moisture in the insulating fluid. Excessive moisture content can reduce the insulating system (both insulating oil and insulating paper) and it can reduce the expected useful equipment life by deteriorating the cellulose insulating material. Moisture can be absorbed into the fluid from tank leaks, produced by cellulose material breakdown (insulating paper aging), or during sample/shipping.

PCB testing

This test uses gas chromatography to determine the amount of PCBs in the insulating fluid for purposes of jurisdictional compliance.

Metals in oil

This test analyzes the presence of typical metals found in fluid filled electrical equipment to provide vital information about fault locations and excessive wear in mechanical components.

Particles in oil

This Test analyzes the presence of 5 ranges of various sized particles in your insulating oil to provide vital information about the severity of moving parts rate of wear. This test is useful with the evaluation of circuit breakers and load tap changers.

Furan analysis

This test result can be correlated to a value to determines degradation of solid insulation (Degree of Polymerization) based on the presence of five furanic compounds: 5-Hydroxy-methyl-furaldehyde, 2-Furaldehyde, 2-Acetylfuran, 5-Methyl-2-furaldehyde, 2-Furyl alcohol. This test can be helpful with determining the amount of life expended from a transformer’s insulating paper for asset management. 
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